The present application relates to a nonaqueous electrolytic solution, a positive electrode and a nonaqueous electrolyte secondary battery using the same. The present application relates to a nonaqueous electrolytic solution which has a high capacity, high in safety even under an overcharge condition and excellent in cycle characteristic and high-temperature characteristic, a positive electrode and a nonaqueous electrolyte secondary battery using the same.
Owing to the remarkable development of a portable electronic technology in recent years, electronic appliances such as mobile phones and laptop personal computers have started to be recognized as a basic technology supporting a high-level information society. Also, research and development on high functionalization of such an electronic appliance are energetically advanced, and the consumed electric power of such an electronic appliance increases steadily in proportion thereto. On the contrary, such an electronic appliance is to be driven over a long period of time, and realization of a high energy density of a secondary battery as a drive power source has been inevitably desired. Also, in view of consideration of the environment, the prolongation of a cycle life has been desired.
From the viewpoints of occupied volume and mass of a battery to be built in an electronic appliance, it is desirable that the energy density of the battery is as high as possible. At present, in view of the fact that a lithium ion secondary battery has an excellent energy density, the lithium ion secondary battery is now built in almost all of appliances.
Usually, the lithium ion secondary battery uses lithium cobaltate for a positive electrode and a carbon material for a negative electrode, respectively and is used at an operating voltage in the range of from 4.2 V to 2.5 V. The fact that in a single cell, a terminal voltage can be increased to 4.2 V largely relies upon excellent electrochemical stability of a nonaqueous electrolyte material or a separator.
For the purposes of realizing higher functionalization and enlarging applications on such a lithium ion secondary battery, a number of investigations are being advanced. As one of them, for example, it is studied to contrive to make a lithium ion secondary battery have a high capacity by enhancing an energy density of a positive electrode active material including lithium cobaltate.
However, in the case where charge and discharge are repeated at a high capacity, in particular, in a high-temperature region, not only an electrolytic solution coming into physical contact with a positive electrode is oxidized and decomposed, a gas is generated to cause defectives such as blister, rupture and liquid leakage of the battery; but at the time of overcharge, the electrolytic solution is decomposed on the positive electrode, and furthermore, deposition of metallic lithium on a negative electrode, an internal short circuit and the like are caused, thereby possibly remarkably impairing the safety.
Then, as a technique for securing the safety at the time of overcharge of a nonaqueous electrolytic secondary battery, there are proposed a method of using a previously installed safety device and a method of imparting resistance to overcharge to a battery itself.
For example, Japanese Patent No. 3061756 discloses a method in which a compound added to an electrolytic solution causes polymerization or the like at the time of overcharge to increase an internal resistance of a battery, thereby protecting the battery. Also, JP-A-10-321258 discloses a method in which a conductive polymer is formed at the time of overcharge to induce an internal circuit within a battery, thereby causing automatic discharge. Japanese Patent No. 3061759 discloses a method in which a gas is generated at the time of overcharge, thereby surely operating an internal current breaking device working at a prescribed internal pressure.
Japanese Patents Nos. 2983205 and 3113652 disclose that a benzene based compound is surely decomposed at a prescribed voltage to generate a gas, thereby enabling a battery to be protected at the time of overcharge. JP-A-2006-73308 discloses that in a nitrogen-containing heterocyclic derivative called a tetrazole, a gas is generated relying upon only a potential but not a temperature.